The present invention relates to a magnet roller suitably used for a developing mechanism portion for supplying a developer to a latent image support such as a photosensitive drum so as to develop an electrostatic latent image on the latent image support in an electrophotographic process using a copying machine, facsimile, printer or the like, and a method of manufacturing the magnet roller.
Conventionally, in an electrophotographic device or electrostatic recording device such as a copying machine or printer, there has been a developing method in which as a developing roller for visualizing an electrostatic latent image on a latent image support such as a photosensitive drum, a magnet roller formed of a resin or rubber magnet is disposed in a rotating sleeve, and a magnetic developer (toner) supported on the surface of the sleeve is supplied on the surface of the latent image support by a so-called jumping phenomenon (which allows the toner to be jumped on the latent image support due to a magnetic force characteristic of the magnet roller), to thereby visualize the electrostatic latent image.
The above described magnet roller has been manufactured by injection-molding or extruding, using a mold around which a magnetic field is formed, pellets of a resin or rubber composition containing magnetic powders of a ferrite or the like mixed in a binder composed of a thermoplastic resin such as nylon or polypropylene or a rubber, into a roller shape, and magnetizing the roller to give a desired magnetic force characteristic to the roller.
With the recent progress of electrophotographic devices and the like, more complex magnetic force patterns have been required for magnet rollers. However, the related art magnet rollers have a limitation to a magnetic force pattern to be designed, and cannot sufficiently meet the above requirement.
For this reason, to increase the degree of freedom in magnetic force pattern of a magnet roller, there has been adopted a method in which a plurality of magnet pieces in which magnetic poles corresponding to a desired magnetic force pattern are magnetized are formed of resin or rubber magnets and are stuck around a shaft, to thereby attain the desired magnetic force pattern.
In this case, to obtain the above magnet piece having a high magnetic force peak, there has been proposed a method of molding the magnet piece using a mold shown in FIG. 2 (Japanese Patent No. 2512025). The mold shown in FIG. 2 is configured that magnetic material members 3a and 3b made from iron or the like are disposed on upper and lower sides with a non-magnetic material member 2 put therebetween, and a cavity 1 is formed between the magnetic material members 3a and 3b. Using such a mold, the magnet piece is formed by injection-molding or extruding the above resin or magnet material in a state in which a coil 4 is applied with a current to form a magnetic field around the cavity 1. In this case, there has been proposed a method in which the above magnet piece is molded using such a mold that both side surfaces and the back surface (opposed to the front surface side) of the cavity 1 are formed of the magnetic material member 3b and the upper magnetic material member 3a is provided with a projecting portion 5 in such a manner that the leading end of the projecting portion 5 is disposed in the vicinity of a specific position of the cavity 1 on the front surface side. In the magnet piece molded using such a mold, as shown by arrows in FIG. 2, magnetic powders are oriented in such a manner as to be converged from both the side surfaces and the back surface side to a specific position on the front surface side, to thereby obtain a high magnetic force peak.
However, in the magnet roller in which a plurality of magnet pieces molded using the mold shown in FIG. 2 are fixedly disposed around the outer periphery of a shaft, the magnetic force peak of each magnetic pole is increased by the above-described orientation characteristic. However, when a low magnetic force is required, the diameter of the magnet piece must be reduced, or when the magnetic force peak is moved to a position offset from the central portion of the magnet piece, the magnet piece must be molded using the mold modified such that the projecting portion 5 provided on the upper magnetic material member 3a is located at a position offset from the central portion of the cavity 1 as shown in FIG. 3. That is, magnet pieces must be molded using various kinds of molds corresponding to the magnetic force patterns necessary for the magnet pieces, to thereby increase the manufacturing cost of the magnet roller formed of the magnet pieces.
In manufacture of a magnet roller, there has been adopted a method in which a magnet roller molded using a mold in which a magnetic field is applied around a cavity is magnetized once and then magnetized again to give a desired magnetic force pattern thereto. However, for the magnet roller formed of magnet pieces molded using the mold shown in FIG. 2 or 3, the position of the magnetic force peak is very restricted due to the above-described orientation characteristic. Accordingly, if the magnetic force peak is moved after formation of the roller in which the magnet pieces are fixedly disposed around a shaft, there occurs a problem in which a magnetic force is significantly reduced. That is, actually, the magnetic force peak cannot be moved after formation of the magnet roller.